High frequency network structure



March 26, 1940. -n5 2,194,543

HIGH FREQUENCY NETWORK STRUCTURE Filed April 13, 1938 2 Sheets-Sheet lFIG. 2B

. A 7 I Fla 3 }4 FIG. 3A FIG. 3B '83[ g 4A F .9 g FIG. 4 FIG-4A g [-7645%4 /Nl/ENTOR By HE. CURT/S Patented Mar. 26, 1940 UNITED STATES v2,194,543 HIGH FREQUENCY NETWORK- STRUCTURE Harold E. Curtis, Orange, N.5., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application April-13, 1938, Serial No.201,763 10 Claims. (c1. 178-44) This invention relates to high frequencyelectrical translating systems and more particularly .toreactive-elements and networks appropriate for oscillatory, terminatingand translation circuits in radio frequency signaling systems.

An object of the invention is to provide a unitary structure thathasfrequency selectivechar acteristics and that is electrically balanced orsymmetrical with respect to ground potential. A more specific object isto provide a structure of this kind that is adapted for use inconjunction with balanced transmission circuits generally and moreespecially with balanced-transmission circuits comprising a pair ofshielding conductors.

Another object is to provide for ready variability of the frequencyselective properties of such elements and networks.

In accordance with the present invention the foregoing objects and otherobjects thatwill appear hereinafter are realized in various structuresutilizing the reactive properties of short sections of transmissionlines. It can readily be shown that a short-circuited transmission line,the length of which is short compared with the length of the wavesapplied to it, has the characteristic of a substantially pureinductance, and

that the magnitude of the inductance is substantially proportional tothe absolute length of the line. In the important case of ashort-circuited transmission line comprising a pair of cylindricalconductors symmetrically disposed within a cylindrical conductingshield, a maximum inductive effect, for a shield of given diameter, isobtained when the spacing ratio is 0.486, that is, when the conductorsare so arranged that 0.486 represents the ratio of the separationbetween each conductor and the axis of the shield on the one hand andthe internal radius of the shield on the other. For such ashort-circuited transmission line the total inductive effect L at anyfrequency for which the length of the line is short compared with thewave-length is:

L=4 log. 0.60 X% abhenries per centimeter (1) where 0 represents theinner diameter of the shield and b the outer diameter of the enclosedconductors. It is evident from Equation 1 that the larger the diameterratio I a that the inductanceis a function of the length of thelinqt...vj

structures adapted for ready adjustment, Figs.

conductors ofline I. Preferably the plane of the 'latter conductors isperpendicular to the axis of shield 2 and Wires 4 so as to preserveelectrical The nature of the present invention will appear more fully inthe following description of various typical preferred embodimentsillustrated in the accompanying drawings.

Fig. 1 shows a shielded pair line with a two- 5 terminal structure inaccordance with the invention branching therefrom; t

, Figs. 2 to 6 show schematically longitudinal sections of modifiedforms of two-terminal network structures, Figs. 2A to 6A showcorrespond- 1o ing cross-sectional views thereof, and Figs. 23 to 613 Irepresent the respectively corresponding equivalent electrical circuits;

Figs. 7 to 11 show schematically longitudinal sections of four-terminalnetwork structuresjin '15 accordance with the invention, and Figs. 7A to11A and Figs. IE to 113 represent the respectively correspondingcross-sectional views and equivalent circuits thereof; 7 I

Figs. 12 and 13 show two-terminal network 12A and 13B represent therespective equivalent circuits thereof, and Fig. 13A is across-sectionalview corresponding to Fig. 13.

Referring now to Fig. l, thereis shown an elemental combination adaptedto introduce a shunt inductance across a balanced transmission line lthat comprises a pair of shielded conductors. The

inductance element is in the form of a balanced transmission linestructure branching from the 30 line I and. it comprises a cylindricalmetallic shield 2, a metallic end cap 3, and a pair of fine wires 4which are at one end attached to the end cap 3 and at the other endconnected through metallic leads 5 of larger diameter to the respectiveand mechanical symmetry. The magnitude of the shunt inductance providedby the arrange ment illustrated in Fig. 1, it will be seen from Equation1, depends on the length of the wires .4 and on the ratio of therespective diameters of the shield land the wires 4. Operatingfrequencies of the order of a me'gacycle per second and higher arecontemplated in this example and in the others that are to follow.Supposing, for specific example, that the frequency is 100 megacyclesper second corresponding to a wave-length of 300 centimeters, it may benoted that if the 50 short-circuited line structure were a quarterwave-length long, '75 centimeters, i. e., it would be anti-resonant andthe electrical equivalent of an inductance and capacitance in parallelrelation,

and that if it were .a half wave-length long it ture shall besuiiiciently less than a quarter wavelength that its electricalequivalent is substantialtion to provide the circuit equivalent of aninly inductive.

Figs. 2 and 2A illustrate a modification of the structure shown in Fig.1 in which the wires 4 are terminated in metallic plates 6 which lieparallel to but spaced apart from the end cap 3. With this arrangementthe equivalent circuit, as

indicated in Fig. 23, comprises the inductance of the shielded wires inseries relation with a capacitance dependent on the size and spacing ofthe plates is. It is a feature of the construction that the capacitanceelements are so disposed that they retain the electrical symmetry orbalance of the structure with respect to ground, and that for anyparticular circuit application connection can be made to the electricalmidpoint of the structure simply by connecting to the center of the endcap 3. It is a feature also that the condition of resonance can beattained by suitably determining the magnitude of the capacitance andwithout resort to sometimes unwieldy lines of half wave-length.

Another modification is illustrated in Figs. 3 and 3A where theequivalent circuit as shown in Fig. 313 comprises an inductance andcapacitance in parallel relation. In this case the incoming leads 5 areterminated in a pair of transversely disposed metallic plates 7 whichcarry on their respective peripheries flanges 8 which are in capacitiverelation with the shield and, along'a diameter, with each other. Withthis combination it is possible to obtain the characteristics of aquarter wave-length line although the actual length of the line is muchless than a quarter wave-length.

Figs. 4 and 4A illustrate how the structure shown in Fig. 1 can bemodified in another direcductance and a resistance in parallel relation.For this purpose a disc s of resistive material is provided whichconstitutes a resistive shunt acrossthe input end of the structure asshown in the equivalent circuit comprising Fig. 4B. Series resistancemay be obtained inthis case and in all other cases herein disclosed byconstituting the wires :3 of a resistive metal or alloy.

Figs. 5 and 5A show'the application of the resistance disc 9 to thedamping of a tunedcircuit structure of the kind shown in Fig. 2. In thiscase the tuned circuit, as shown in Fig. 53, comprises balancedinductance and capacitance as in Fig. 2B with a resistive shunt acrossthe input terminals. The structure illustrates also an applicationof thefact that a greater inductance .per unit length may be obtained byincreasing the diameter of the shield as well as by decreasing the sizeof the wires 4.

Figs. 6 and 6A show an alternative form of series tuned structure havingan equivalent circuit of the kind shown in Fig. 6B. The capacitanceelements in this case are provided by a pair of metallic plates 7'terminating the input leads, in combination with juxtaposed metallicplates 6' respectively connected to the wires 4.

A-balanced four-terminal translation circuit is shown in Figs. '7 and 7Aand the equivalent circuit thereof is shown in Fig. 7B. In thisembodimerit a transverse resistance disc I!) is provided near the centerof the structure and the shielded wires 4 arebroken and the fourproximate ends thereof terminated in metallic plates l6, each of whichis in capacitive relation with the disc l9.,

The four sections of wire provide the four inductance elements of. theequivalent circuit, the plates it provide the four series capacitancesand the disc l9 constitutes the shunt resistance. The translationstructure may be interposed in a twowire shielded line,for example, andutilized for selectively passing a substantially single frequencydetermined by the relative magnitudes of the inductive and capacitiveelements, or a. wider band of frequencies dependent on the dampingeffect of the shunt resistance l9.

Two other forms of translation structures are shown in Figs. 8 and 8Aand 9 and 9A,respec-* tively, with respective equivalent circuits as'inIfigs. 8B and 9B. In Fig. 8 a shunt capacitance is provided at theelectrical mid-point of the structure by a capacitance structure of thekind described with reference to Fig. 3, whereas in Fig. 9 two suchcapacitive structures 8 and 8, are utilized, one at the input terminaland the other at the output terminal. Both structures may be utilized,for example, aslow-pass filters.

In Figs. 10 and 10A is shown a four-terminal network structure which isequivalent to two series inductance elements each shunted by acapacitance as indicated in Fig. 103. The series inductance is obtainedby means of the line wires 6 as hereinbefore, and the shuntingcapacitance by metallic cup-like members iii each surrounding a portionof the wires l and each terminated in an annular lip or internal flangeH disposed in capacitive relation with the corresponding portion ofanother axially aligned member Ii]. The

amount of series capacitance is largely, determined by the areas andspacings of the lips I I.

Figs. 11 and 11A illustrate a modification of the network structureshown in Fig. 10, the modification being that the wires 4 are eachinterrupted at their mid-points by a pair of juxtaposed plates l2 whicheffectively introduce a series capacitance in the inductive arms of thenetwork as appears in the equivalent circuit shown in Fig. 113. Theequivalent circuit is otherwise in Fig. 10B and is similarly derived.

Whereas the network structures shown hereinbeforehave been described asbeing of the fixed type, Figs. 12 to 13A show two preferred forms ofstructures in which the reactance elements are capable. of readyadjustment for any. particular circuit application. The structure shownin Fig.

the same as that 12 provides inductance and capacitance in seriesrelation as indicated in Fig. 12A. From theinput leads .i-the pair ofline wires 4 extend to the right and are terminated in incapacitive'relation end cap l3 which may be adjusted to vary itsseparation from the plates 8 and thereby to vary, the magnitude of theseries capacitance in the circuit. To vary the effective length of thewires 4 and thereby to vary the amount'of series inmetallic plates 6disposed ductance in the circuit, metallic cylindrical caps M areprovided, each adapted to slide over the end of one of the input leads 5with one of thewires 4 extending through a central'aperture in the endof the cap. A rod 55 of insulating material mechanically connected withthe caps serves to adjust their positions.

The structure shown in Figs. 13 and 13A has the characteristics of acapacitance and inductance in parallel relation. The inductive Wires 4are permanently connected between the input leads 5 and the-metallic cap3, and a metallic piston 23 having openings therein through-which withan adjustable metallic the wiresi'4 pass serves to: adjustthe, effectivelength of the line and thereby to vary. the amount of series inductancein circuit. An insulating rod .l'l extending from the piston 23 throughan aperture in-the end cap 3 can be used to efiect the necessaryadjustments. The shunt capacitance element comprises a structure of thegeneral kind illustrated in Fig. 3, but arcuate metallic members 2c aredisposed between each metallic plate and the shield 2. Respectiveradially disposed rods E8 of insulating material extend throughapertures in the shield and are connected with the respective arcuatemembers 20 so that the relative positions thereof between the flanges 8and the shield 2 may be adjusted. The total amount of shunt capacitanceis dependent on the positions of the members 20.

Whereas only a few typical embodiments have been illustrated anddescribed in this specification, it will be understood that theinvention comprehends such other embodiments as come within the spiritand terms of the appended claims.

What is claimed is:

1. In a system for the transmission of ultrashort electromagnetic wavesover a pair of conductors enclosed within a metallic sheath, a 10-calized reactive circuit therefor comprising a metallic chamberelectrically associated with said sheath and a pair of conductors withinsaid chamber that are electrically connected with the first-mentionedpair of conductors and that are short as compared with the length ofsaid waves, the ratio of the transverse dimensions of said chamber tothe diameter of the conductors enclosed thereby being of a greater orderof magnitude than the corresponding ratio of the transverse dimensionsof said sheath and the conductors enclosed thereby, said terminationhaving the electrical properties of an inductance electrically balancedwith respect to ground potential.

2. A combination in accordance with claim 1 including in addition abalanced capacitive element, said element comprising a pair of metallicmeans each electrically connected with a respective one of saidconductors within said sheath and having juxtaposed surfaces providing alumped capacitance, said metallic means being symmetrically disposedwith reference to said chamber to preserve the electrical balance of thesystem.

3. A combination in accordance with claim 1 including in addition atransverse resistive barrier disposed across said chamber'and inelectrical contact with both of the conductors enclosed thereby.

4. In a high frequency transmission system, a network structure havingthe characteristics of inductance and capacitance in series relationsymmetrically disposed with respect to ground, said structure comprisinga section of shielded pair, a metallic cap closing one end of the shieldthereof and a pair of metallic plates each in capacitive relation withthe inner surface of said cap and connected respectively to theproximate terminals of the conductors comprising said pair, whereby aseries inductance eifect is derived from said conductors and a seriescapacitive efiect from said metallic plates and cap, all of saidconductors and plates being so constructed and arranged as to provideelectrical and mechanical symmetry.

5. In a high frequency transmission system, a network structure havingthe characteristics 01 inductance and capacitance in parallel relationand symmetrically disposed with respect to ground, said structurecomprising a section of shielded pair,"a metallic cap closing one end ofthe shield thereof and constituting a short circuit for the conductorscomprising said pair, a pair of metallic plates each disposedtransversely and connected to one of said conductors, a peripheralmetallic flange on each said plates, said flanges length whereby abalanced shunt inductive effect is obtained.

, 6. A combination in accordance with claim comprising in addition ametallic plate interposed between each-of said flanges and said shield,and

means for adjusting the relative distances between the flanges and theplates and between the plates and the shield, whereby the magnitude ofthe equivalent shunt capacitance can be controlled.

'7. In a transmission line comprising a shielded pair for transmissionof waves at frequencies above a megacycle per second, a low-pass filterstructure incorporated in said line, said filter structure comprising alength of said line short compared with the operating wave-length, inwhich the conductors comprising said pair are of substantially reduceddiameter so as to obtain an enhanced series inductive eifect, and atleast one shunt capacitance structure within said length of line, saidcapacitance structure comprising a pair of metallic plates transverselydisposed and connected with a respective one of said conductors, and aperipheral metallic flange on each of said plates, said filter structurebeing so constructed and arranged as to preserve the electrical balance,with respect to ground, of the line in which it is incorporated.

8. In a line comprising a shielded pair for the transmission ofultra-high frequency waves, an electrically balanced network structureincorporated in said line and having electrical properties of a networkcomprising a capacitance in one series arm and an inductance in aparallel-connected series arm, said structure comprising a portion ofsaid line, the length of which is small compared with the operatingwave-length, in which the conductors comprising said pair are of greatlyreduced'diameter whereby a series inductive effect is obtained, and apair of metallic cup-like members coaxially disposed around each of saidconductors with their open ends juxtaposed, each of said members havingat its open end an internal metallic flange in capacitive relation withthe flange of the other member comprising the pair, whereby a seriescapacitive effect dependent on the area and spacing of said flanges isobtained.

9. In an ultra-high frequency transmission system, a transmission linecomprising a shielded pair and a network structure at the end thereofhaving the characteristics of inductance and capacitance in seriesrelation symmetric with respect to ground potential, said structurecomprising a section of said shielded pair, the length of which is smallcompared with the operating wave-length, in which the conductorscomprising said pair are of greatly reduced diameter, a metalic barrierclosing the end of the shield, a pair of metallic plates connected tosaid conductors and in capacitive relation with the inner face of saidbarrier, said barrier being adapted for longitudinal adjustment tocontrol the series Capacitance introduced by said plates, a metallicextension coaxially disposed on e'achOf-said conductors and arranged toslide over the ends of the conductor portions of normal diameter wherebythe efiective length of said conductors of reduced diameter and thecorresponding series inductive effect-can be adjusted independently ofthe adjustment of series capacitance.

10. In a high frequency transmission system, a shielded pair comprisingconductors of normal diameter and a network structure at the end thereofhaving the electrical characteristics of inductance and capacitance inparallel shunt relation and in balanced relation with respect to ground,said structure comprising a section of shielded pair the length of whichis small compared with the operating wave-length, the conductors' ofsaid section being of small diameter as compared with said conductors ofnormal diameter, short-circuiting means comprising a metallic pistonhaving apertures through which said. conductors of small diameter passand means'for structure independently of the adjustment of-Tfi theinductance.

HAROLD E. CURTIS.

